Crystalline materials can be identified by their x-ray diffraction (XRD) pattern, which is unique to each material and can serve as its ‘fingerprint’. Among materials that present security hazards are explosives, illegal drugs and spores (e.g. Anthrax spores). The patent suggests a remote detection method to identify suspect materials according to their XRD pattern. The suspect material in a Volume of Interest (VOI), may be recognized by lower stage detection systems, such as: X-ray imaging system, Average density identification by multiple energy X-ray system, NMR (MRI), NQR, IR imaging, millimeter wave imaging, THz imaging, etc.
If monochromatic X-rays impinge upon a polycrystalline sample with randomly oriented crystallites, then some of the crystallites fulfill Bragg's law with respect to the x-ray beam. Thus, all reflections belonging to a particular lattice plane are distributed upon the mantle of a circular cone, of which the x-ray beam is the axis and the aperture angle is 4θ. An x-ray sensitive film or an X-ray detector placed perpendicularly to the x-ray beam will thus record a diffraction image comprising a series of a series of concentric circles. The Bragg angle is given by equation (1):
                    θ        =                              1            2                    ⁢          arctan          ⁢                      D                          2              ⁢              x                                                          (        1        )            where D is the diameter of a diffraction ring, and x is the distance between the sample and the film.
A variety of X-ray detectors have been presented in the art. Digital Radiography X-ray recording comprises the steps of capturing X-ray photons and converting the recorded signal to an electrical signal. These systems are intrinsically pixelated to form either a pixelated array or a continuous array with a moving “pixelated-bridge” (“pixelated-bridge” means a one-dimensional or very narrow two-dimensional scanning pixel array). The detectors may be divided into two main groups, namely direct and indirect detectors. Direct detectors use a plurality of photo-conducting materials such as silicon, germanium, selenium, CdTe, CdZnTe, PbI2 or HgI2, and are adapted for directly converting x-ray energy into electric charge utilizing TFT (Thin Film Transistor), CMOS (Complementary Metal-Oxide Semiconductor) technology or any other type of substrate whether continuous or pixelated array. This charge can be then captured, stored and recorded. Indirect detectors use a plurality of scintillator materials such as NaI, CsI or Gd2O2S, to convert the x-ray energy into visible or UV light, which must be optically coupled to a photosensitive device, e.g. a photo-diode array or charge coupled device (CCD). This photo sensor then converts the light into electric charge, which can be captured, stored and recorded.
Indirect conversion is a two-step process wherein X-rays are first converted by a means of a scintillator or phosphor material to lower energy, e.g. visible light, photons that are then collected and converted into an electric charge. Commercially available products of GE Medical Equipment Inc. comprising an amorphous silicon flat panel with a Cesium Iodide scintillator is an example of such a technique.
Direct conversion is a single conversion step process. At least three types are known in the art: TFT, CMOS and Continuous plates. The TFT is coated with a photoconductor, wherein the detector uses direct conversion of x-ray energy into electrical signals. No light-emitting materials, intermediate steps and/or additional processes are required to capture and convert the incident x-ray energy. The commercially available products of Hologic Inc. are an example of this type, wherein an amorphous selenium photoconductor is used. Continuous plates are scanned by means of a moving pixellated-bridge. A selenium-based sensor is used to convert incident X-rays into an electric charge image. The charge image is transformed into a digital image using this bridge, which eliminates the need for costly and often-problematic active matrix arrays. The commercially available products of Edge Medical Devices Ltd. and its Scanned Matrix Array Readout Technology (SMART) uses this technology.
US Pat. Application 2001/0033636 to Hartick et al. (now abandoned) discloses a method and apparatus for determining a material of a detected item and deals with a specific method of defining a VOI by means of calculating the average density of the detected volume and a correlation between the VOI and the XRD shots.
U.S. Pat. No. 6,839,406 to Ries et al. presents a method and an apparatus for detecting unacceptable items in objects, such as in luggage, wherein a detector apparatus, functioning as a second detector stage is divided into a lower level testing stage and a higher level testing stage. This invention deals with a novel and yet specific energy dispersion method of XRD.